JPS61292617A - Light frequency modulation method - Google Patents

Abstract

PURPOSE:To realize light frequency modulation without using direct modulation of a light source by switching light from >=2 light sources of different oscillation wavelength conforming to information signals, and sending out different wavelength for each information. CONSTITUTION:Light emitted from the first light source 1 and the second light source 2 is led respectively to input ports A and B of a light switch 5 by the first single polarization fiber 3 and the second single polarization fiber 4. Voltage impressed to the light switch is controlled by a signal generator 6. Thereby, the port A input is emitted from a port C, and the port B input is emitted from the port C at the time of transmitting a mark signal, and the port A input is emitted from the port D and the port B input is emitted from the port C at the time of transmitting a space signal. Accordingly, the output of the port C and port D differs in frequency according to information signals and the frequency shift keying (FSK) is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to optical communications, and particularly to a method of modulating the frequency of transmitted light.

(Prior Art and its Problems) In recent years, as an optical communication system, a coherent optical transmission system that uses information on the frequency and phase of light has been studied in various places. In particular, in the case of a frequency shift keying (FSK) optical heterogain optical communication system that uses optical frequency information, multilevel transmission is possible. It has the characteristic of being able to achieve high reception sensitivity.

Normally, when performing frequency modulation in optical communications, direct modulation is used in which the injection current of a semiconductor laser is slightly changed to change its oscillation frequency.

However, semiconductor lasers generally have a spectral broadening of 9, and the PgK optical heterogain communication system has a problem in that this spectral broadening causes deterioration of reception sensitivity. Furthermore, when performing multi-value conversion, there is also the problem that the frequency interval between the multi-signals is limited by this spectral broadening υ, which limits the range in which multi-value conversion can be performed.

A method of adding an external mirror to a semiconductor laser has been considered as a method for solving the problem of 9 spectral broadening of the laser. Shikaji Tsuken Jitsugou Volume 31 No. 12 1982
It is known that the frequency modulation efficiency of a semiconductor laser with an external mirror added is significantly inferior to that of a single semiconductor laser, as shown in the paper “Direct frequency modulation characteristics and FM noise characteristics of semiconductor lasers 1” by Shiraki et al. For this reason,
There has been a problem in that it may not be possible to obtain a necessary frequency shift when performing speed-controlled transmission using binary frequency modulation or when performing multi-value frequency modulation.

(Objective of the Invention) Therefore, the object of the present invention is to solve the above problems and provide a method for realizing optical frequency modulation without using direct modulation of the light source even when using a light source with a high spectral purity. be.

(Structure of the Invention) The optical frequency modulation method of the present invention uses at least two or more light sources with slightly different oscillation frequencies, the light from these light sources as input light, and a control signal). It is equipped with an optical switch that can selectively output one of them, and changes the output light from the optical switch according to the information signal to change the frequency of the transmitted light and perform frequency modulation of the signal light. This is achieved by

(Operation and Principle of the Invention) The present invention switches light from two or more light sources with different oscillation wavelengths in accordance with an information signal, and transmits a different wavelength for each piece of information. In this case, the transmitted light is subjected to FSK modulation in accordance with the information signal. The operation and principle of the present invention will be explained in detail below, taking as an example a case where a 2×2 waveguide type optical switch is used as the optical switching element.

Figure 2 is a diagram showing the configuration of a 2X2 waveguide type optical switch.
FIG. 3 is a diagram showing the relationship between the optical output and the applied voltage of this optical switch.

Let us consider a case where light from light sources with slightly different wavelengths is incident on the light switch (Boat) and (Bo)H of this optical switch. Here, when no voltage is applied to the optical switch, the light of wavelength λi incident from boat A is directly emitted from boat C. On the other hand, when the switching voltage Vs is applied to the optical switch, only the light having the wavelength λ2 incident from the boat B is emitted from the boat C. On the other hand, the light that is not emitted from boat C is emitted from boat D. Therefore, if the voltage applied to the optical switch is changed by the switching voltage in accordance with the information signal, the frequency of light output from each output port will be changed in accordance with the information signal.

(Embodiment) FIG. 1 is a block diagram for explaining a first embodiment of the present invention.

The light emitted from the first light source 1 and the second light source 2 is input to the optical switch 5 through a first single-polarized fiber 3 and a second single-polarized fiber 4, respectively. ) led to H. Here, as the optical switch 5, a 2×24 waveform optical switch fabricated on a lithium niobate (LiNbO5) substrate was used.

The voltage applied to the optical switch 5 is controlled by a signal generator 6. As a result, when sending a mark signal, the port human input is emitted from boat C, the baud) B input is emitted from boat C, and when sending a space signal, the baud) A input is emitted from baud) D, and the boat B input is emitted from boat C. It is emitted from C. As a result, the frequencies of the outputs of the baud C and baud D are changed in accordance with the information signal, and so-called frequency shift keying (FSX) modulation is performed. Boat C,
The outputs of the boat D are input to the first transmission line 7 and the second transmission line 8, respectively, and the information is propagated to different terminal stations.

In this embodiment, as the first light source 1 and the second light source 2, InGaAsP distributed #recirculated semiconductor lasers having a wavelength of 1.5 μm and having an increased spectral width of 100 kHz and spectral purity by adding an external mirror were used.

Here, modulation is performed at F8 on the transmitting side, but considering that this light is subjected to heterodyne detection on the receiving side, at least the frequency shift of the modulated light must be kept constant. Therefore, in this embodiment, the oscillation frequency difference between the two semiconductor lasers was controlled to a constant value using the following control system, and the frequency shift during FSK modulation was kept constant. First, the output lights from the back surfaces of the first light source 1 and the second light source 2 are incident on a coupler 9 composed of a single mode fiber and multiplexed. Signal detection was performed. Through this heterodyne detection, a beat signal having a frequency corresponding to the frequency difference between the two semiconductor lasers is obtained, and the deviation of this beat signal frequency from the set value is detected by the frequency discriminator 11.
The error signal obtained here was fed back to the injection current of the second light source 2 via the bias control circuit 12. With this control system, the oscillation frequency difference between the two lasers was always kept constant.

In this example, information was transmitted at 400 Mb/s. In order to obtain high optical reception sensitivity on the receiving side, it is necessary not to widen the reception band too much, and to set the amount of frequency shift in F8 within a range that does not cause interference between each frequency component of the demodulated signal. . Therefore, in this embodiment, considering the transmission speed, the frequency interval between the first light source l and the second light source 20 is set to 400 MH.
Control was performed so that z. The modulation at F8 of 400 Mb/s is the switching voltage of optical switch 5.
The crosstalk between the two laser beams at 400 Mb/s non-return with an amplitude of 'V was suppressed to less than -20 dB. When we investigated the receiving sensitivity by optical heterodyne frequency differential detection of the light propagated through the first transmission line 7, we found that the spectral width of the light source was sufficiently narrow, so there was no floor in the code error rate characteristics, and the error rate was 10.
1, a high value of -47 dBm was obtained.

FIG. 4 is a block diagram for explaining a second embodiment of the present invention.

In this embodiment, modulation is performed to four-value F8. The optical switch 5 used in this example is a LiNbO34X 1 optical switch that selects one of the four input lights by changing the applied voltage.
are selectively emitted. This light switch 5
The light from the first light source 1, the second light source 2, the third light source 13, and the fourth light source 14 is input into the first single-polarized fiber 3 and the second single-polarized fiber, respectively. The fiber 4, the third single polarization fiber 15, and the fourth single polarization fiber 16 were used for guiding. Controlling the oscillation frequency difference of the light source is the first step.
The control system 21, the second control system 22, and the third control system 23 were used. The configuration of each control system is similar to the configuration of the control system described in the first embodiment.

In this example, 400 Mb/s information is also used.
Value FSK modulation was performed. In this case, since the output light can pass through four oscillation frequencies, modulation can be performed at a speed of 200 Mb/s'. Therefore, the control signal to be applied to the optical switch 5 was created by converting the signal from the signal generator 6 into a four-value signal using the code converter 17. In this case, it is sufficient if the frequency difference between each light source is 200 MHz, and in this embodiment, the first light source 1 is used as a reference, and the second light source 2 is
00MHz high frequency, third light source 11 is 400MHz
z high frequency, the first, second and third control system 21.22.2 so that the fourth light source has a 600 MHz high frequency
The oscillation frequency was controlled in step 3.

In this example as well, the reception feeling was evaluated after the output of the optical switch 5 was transmitted through the transmission line 7. In this example, modulation is performed by the optical switch 5, so distortion due to f-key hardly occurs during modulation, and deterioration in sensitivity due to multilevel modulation is hardly observed. Ta. In this embodiment, modulation and multi-leveling are performed to 4-level F8, so the reception sensitivity is improved by 2 dB compared to the first embodiment.
A high optical reception sensitivity of -49 dBm was obtained with an error rate of lO''-11.

In addition to the above-described embodiments, various modifications of the present invention can be considered. For example, it is possible to increase the degree of multileveling according to the scale of the optical switch 5. Furthermore, the output can be up to the same number as the number of output ports of the optical switch 5, and it is possible to use it for local area networks and the like. Furthermore, by combining a multi-wavelength integrated laser and the optical switch 5, it is also possible to construct a compact transmitter. Further, as a method of multi-value conversion, it is also possible to increase the degree of multi-value conversion by subjecting the light source 1, light source 2, etc. to multi-value frequency modulation and then switching those lights with the optical switch 5. As a light source, in addition to semiconductor lasers, gas lasers, solid-state lasers, etc. can be used, and as a method of controlling the frequency difference between the light sources, temperature control, resonator length control, etc. can be used depending on the laser used. I can do it.

[Brief explanation of the drawing]

Fig. 1 is a block diagram for explaining the first embodiment of the present invention, Fig. 2 is a diagram showing the configuration of a 2x2 optical switch of LiNb01, and Fig. 3 is a diagram showing the configuration of the optical switch of Fig. 2. FIG. 4 is a diagram showing the relationship of light output to voltage.
FIG. 2 is a block diagram for explaining an embodiment of the present invention. In the figure, 1.2, 13.14... light source, 3, 4.15.16...
...Single polarization fiber, 5... Original switch, 6...
Signal generator, 7, 8. ...Transmission line, 9-・Coupler,
10... Photodetector, 11... Frequency discriminator, 12.
...bias control circuit, 17... code converter, 21,
22.23... Control system.

Claims (1)

[Claims] At least two or more light sources with slightly different oscillation frequencies and an optical switch that takes the light from these light sources as input light and can selectively output one of the input lights according to a control signal are prepared, and an information signal is generated. An optical frequency modulation method, characterized in that the light transmitted from the optical switch is switched depending on the optical switch.